High refractive index glass beads are highly essential material in the manufacture of reflective sheeting used for traffic signs for night traffic safety, vehicle stickers, reflective cloth, reflective paint etc. For road signs erected on high speed highways, reflective sheeting of high brilliance is specially necessary, but in the manufacture of such reflective sheeting of high brilliance, glass beads with high refractive inices are essential. In particular, this invention is for supplying superior quality high refractive index glass that is the source of the glass beads used in high brilliance reflective sheeting.
Since reflective sheeting having high reflective brilliance must maintain its high brilliance even in rainy weather, it is essential that the reflective sheeing have a flat surface. Glass beads are closely packed in a single layer facing the surface within the thin binder (sheet) of transparent synthetic resin (colored or colorless), but said synthetic resin decreases the effectiveness of the glass to act as a spherical lens. Therefore glass beads for high brilliance reflective sheeting requires specially high refractive index glass, for example, for synthetic resin having a refractive index of 1.5 glass with refractive index of about 2.17 and over, and for synthetic resin having a refractive index of 1.45 glass with refractive index of about 2.1 and over, are required.
This invention concerns the compositions of high refractive index glass with refractive indices of over 2.15 for making glass beads of 0.03 to 0.1 m/m diameter that are suited for the manufacture of high brilliance reflective sheeting, with a special feature that PbO is not used in the composition.
Compared to ordinary glass, as unusually high refractive index glass beads for high brilliance reflective sheeting are easily subject to a phenomena of devitrification during the productions stages, special consideration must be given during the production process and also use glass compositions that do not easily cause devitrification. The best means of preventing devitrification during the production process of the glass beads is to chill the beads as rapidly as possible, but there is a limit to that also.
In the past, a high PbO content glass was used for producing glass beads possessing refrative indices of about 2.15 and over that were used for high brilliance reflective sheeting, but from the health standpoint, the use of large amounts of PbO, not only during its manufacturing stages but also in the use of the products, is not desirable.